In recent years, electric appliances and electronic apparatuses are becoming increasingly lightweight and compact. Circuit boards, used as components of electric appliances and electronic apparatuses, are also required to be miniaturized and given higher densities because of this trend.
To achieve the miniaturization of circuit boards, polyimide films have been widely used as materials in such boards because they are inexpensive and have excellent physical properties such as electrical properties, thermal resistance, flexibility, etc. They are used for flexible printed circuit boards, tape-automated bonding tapes, chip-on-film and other substrates.
When used as materials for substrates such as flexible printed circuit boards (hereinafter abbreviated as “FPC”), tape-automated bonding tapes (hereinafter abbreviated as “TAB”) and chip-on-films (hereinafter abbreviated as “COF”), polyimide films are usually adhered to copper foils using adhesives such as epoxy resin, and formed into a laminate. However, such laminates do not fully exhibit the inherent characteristics of polyimides because the thermal resistance and other properties of the adhesives are lower than those of the polyimide.
Some methods for producing laminates without using poorly thermal-resistant adhesives have been suggested. One such method forms a polyimide-copper foil laminate by applying a polyamic acid solution onto a copper foil and drying the solution to convert it to the corresponding polyimide. Another such method forms a polyimide-copper foil laminate by the thermocompressive bonding of a polyimide film to a copper foil. Adhesion between the polyimide layer and copper foil in these polyimide-copper foil laminates is low. Therefore, it is difficult to use a copper foil with a smooth surface (low surface roughness) for such laminates, which is necessary for fine pitching and to handle high frequencies. In addition, the problem is reported in terms of deteriorated heat resistance in such laminates.
Kobunshi Ronbunsyu (Japanese Journal of Polymer Science and Technology) 2000, vol. 57, No. 4, 233 describes the following: when a polyamic acid is directly applied onto a base material, e.g., a copper foil, to allow conversion to the polyimide, copper ions tends to migrate into the polyimide near the copper foil interface, thereby the insulation of the polyimide layer may be decreased. Increasing the thickness of the polyimide layer to ensure sufficient insulation, however, is unfavorable because it prevents an increase in density of electronic circuitry. Therefore, the development of a metal foil laminate which can effectively inhibit the migration of copper ions in a thin film is required.
Polyamideimide-copper foil laminates are known, which are produced by forming a conventional polyamideimide film layer on a copper foil by coating method, thermocompressive bonding method or the like. The adhesion of a polyamideimide layer to a copper foil is superior to that of a polyimide. However, its adhesion is still insufficient. In addition, such laminates have problems with high water absorption, low dimensional stability and poor electrical insulative properties such as dielectric constant. Therefore, such laminates are not suitable as materials for substrates such as FPC, TAB, COF, etc.